Add a LangChain chat model integration for the orchestration service

[christophmeyer]

Checklist

• My issue is not an AI Core service issue, but an SDK issue.
• My issue is a Python related issue, not a JS/TS or Java related issue.
• My issue is not already reported or documented.

Describe the Problem

The SDK's orchestration service (v1 gen_ai_hub/orchestration/ and v2 orchestration_v2/) has no LangChain integration, LangChain support exists only for the proxy provider clients.

Propose a Solution

Provide a LangChain BaseChatModel wrapper backed by the orchestration service (analogous to the existing proxy/langchain/ wrappers).

Describe Alternatives

No response

Affected Development Phase

Development

Impact

Impaired

Timeline

No response

Additional Context

No response